Electronic circuits often need to have circuits which can alter the common mode level of signals. Purely as an example, in current mode logic (CML) circuits, voltages are typically referenced from the most positive supply voltage. However, an analog circuit connected to such a digital CML circuit may need to have its threshold voltages referenced to the negative supply. As a result, at the digital/analog interface, there is a need for a circuit which shifts the signal level between the two voltage supply rails.
More generally, there may be a need to shift the level of a signal to an arbitrary fixed or controllable level.
JP-A-6-260925 discloses a level shift circuit, in which first and second input terminals form a differential input, for receiving a differential input voltage, and are connected to the bases of first and second NPN transistors. The collector terminals of these transistors are connected together, and the emitter terminals are connected to respective output terminals which supply a differential output voltage. The emitters of these transistors are further connected through respective resistors to respective halves of a current mirror circuit. The common mode level of the output signals (i.e. the average of the signals) is determined by the values of components in the circuit, for example the resistance values of the resistors.
An object of the present invention is to provide a level shifting circuit which, in preferred embodiments, can provide a fixed output common mode level which is independent of the supply voltage and the input common mode voltage.